If you have ##\Phi(x,y)=0##, when you differentiate it, you get
$$\frac{\partial\Phi}{\partial x}dx + \frac{\partial\Phi}{\partial y}dy = 0.$$ Compare that to what you have
$$(-2xy)\,dx + (5y^4-x^2)\,dy = 0.$$ How would you (partially) recover ##\Phi## from M(x,y) or N(x,y)?

If you have ##\Phi(x,y)=0##, when you differentiate it, you get
$$\frac{\partial\Phi}{\partial x}dx + \frac{\partial\Phi}{\partial y}dy = 0.$$ Compare that to what you have
$$(-2xy)\,dx + (5y^4-x^2)\,dy = 0.$$ How would you (partially) recover ##\Phi## from M(x,y) or N(x,y)?